1. Field of the Invention
This invention relates to a semiconductor device which necessitates current stricture, and a method for its fabrication.
2. Description of Prior Art
A semiconductor laser is representative of a semiconductor device which requires current stricture or restriction. Owing to its high monochromaticity, directivity, etc., much is expected of the semiconductor laser in its applications to optical communication, optical memory, and others. Such semiconductor lasers have been able to lower a threshold value current density by limiting a current flowing region with such a construction as shown in FIG. 1 of the accompanying drawing, and to oscillate in a single mode.
More specifically, in FIG. 1, a reference numeral 1 designates an n-type GaAs substrate, a numeral 2 refers to an n-type GaAlAs layer, 3 denotes a GaAs active layer, 4 a p-type GaAlAs layer, 5 a p-type GaAs layer, 6 a positive electrode, 7 a negative electrode, 8 refers to protons, and 9 represents a high resistance region. This semiconductor laser is formed by laminating the semiconductor layers 2, 3, 4 and 5 on the substrate 1, and then irradiating protons 8 onto the laminated layers with the positive electrode 6 in a stripe pattern using masking, thereby providing on top of the laminated layers the high resistance region 9. In such a construction, electric current flowing across the two electrodes is restricted by the high resistance region 9, and is concentrated on the stripe-patterned region, on account of which there can be obtained luminescence having low current density and good monochromaticity.
In the above-described fabrication of the semiconductor device, protons are irradiated from the topmost surface of the laminated semiconductor layers, on account of which the region to be restricted becomes broader as it goes down in the depthwise direction, as shown in FIG. 1, with the consequent disadvantage such that the current restricted effect of the resulting semiconductor laser is not enough.
FIG. 2 illustrates another structural example of the conventional semiconductor laser. In FIG. 2, 11 designates an n-type Ga.sub.1-x Al.sub.x As layer (0&lt;x.ltoreq.1), 12 refers to a GaAs active layer, 13 denotes a p-type Ga.sub.1-x' Al.sub.x' As layer (0&lt;x'.ltoreq.1), 14 a p.sup.+ -type GaAs layer, 15 an n.sup.+ -type GaAs substrate, 16 and 17 denote electrodes, and 18 a buried layer. In this construction of the semiconductor laser, the buried layer 18 has a smaller refractive index, and a higher resistance, than the GaAs active layer 12, so that it functions as the current restricting layer, and, at the same time, it confines light within the GaAs active layer 12, thereby causing the semiconductor laser to oscillate with good efficiency and with a low current density.
However, even in such a construction as illustrated in FIG. 2, since the buried layer 8 is made of a material having a composition closer to that of the active layer or each of the laminated layers, values of the refractive index and the resistance to be obtained are limited, so that the current restriction and the light confinement are not satisfactory. In addition, since the buried layer 8 was formed by use of an n-type Ga.sub.1-y Al.sub.y As (y&gt;x, y&gt;x') or a high resistance type GaAs, which is re-grown by the liquid-phase epitaxial method, the conditions for the re-growth were very stringent and the fabrication of the semiconductor laser was disadvantageously difficult.